1. Technical Field
This invention pertains to the field of wavefront sensing methods and devices, and more particularly, wavefront sensing methods and devices used to measure the optical quality of an optically transmissive system or device, for example, an optical component such as a lens.
2. Description
A light wavefront may be defined as the virtual surface delimited by all possible rays having an equal optical path length from a spatially coherent source. For example, the wavefront of light emanating from a point light source is a sphere (or a partial sphere where light from the point light source is limited to emission along a small range of angles). Meanwhile, the wavefront created by a collimating lens mounted at a location one focal length away from a point source is a plane. A wavefront may be planar, spherical, or have some arbitrary shape dictated by other elements of an optical system through which the light is transmitted or reflected.
A wavefront analysis system, including a wavefront sensor, may be used to measure characteristics of an optically transmissive system by detecting the wavefront of light emerging from the system and comparing it to some expected ideal wavefront (planar, spherical, etc.). The optically transmissive system might be a single component or may be very complex, such as a transmissive optics system, (e.g., a collimating lens; ophthalmic lens). The differences between the ideal expected wavefront and the actual measured wavefront are caused by optical aberrations of the system under test (SUT).
A number of different wavefront sensors and associated methods are known. Among these are interferometers and the Shack-Hartmann wavefront sensor. Each of these systems will be described briefly below. A more detailed discussion of wavefront sensing techniques may be found in xe2x80x9cIntroduction to Wavefront Sensors,xe2x80x9d 1995, Joseph M. Geary, SPIE Press.
Interferometers
An interferometer is an instrument that uses interference of light waves to detect the relative wavefront difference between a test light beam and a reference beam. Interferometric methods of sensing a wavefront are highly sensitive but very limited in dynamic range. A typical interferometer can only directly measure optical path differences of less than one wavelengthxe2x80x94a 2xcfx80 phase ambiguity exists beyond the one wavelength point. If the optical path difference is greater than one wavelength, then the correct phase difference is often inferred computationally using phase unwrapping techniques. However, real optical configurations can be constructed where these techniques are likely to fail. Other limitations of interferometric techniques include the necessity of relative stability of the reference and test beam paths. This means that any vibration in the test instrument leads to a degradation of the measurement accuracy.
Shack-Hartmann Wavefront Sensors
A Shack-Hartmann wavefront sensor is a device that uses the fact that light travels in a straight line, to measure the wavefront of light. FIG. 2 shows a basic configuration of a Shack-Hartmann wavefront sensor 200. The Shack-Hartmann wavefront sensor 180 comprises a lenslet array 182 that breaks an incoming beam into multiple focal spots 188 falling on an optical detector 184. Typically, the optical detector 184 comprises a pixel array, for example, a charge-coupled device (CCD) camera. By sensing the positions of the focal spots 188, the propagation vector of the sampled light can be calculated for each lenslet of the lenslet array 182. The wavefront can be reconstructed from these vectors.
However, Shack-Hartmann wavefront sensors have a finite dynamic range determined by the need to associate a specific focal spot to the lenslet it represents. A typical methodology for accomplishing this is to divide the detector surface into regions (called xe2x80x9cAreas Of Interestxe2x80x9d [AOIs]) where the focal spot for a given lenslet is expected to fall. If the wavefront is sufficiently aberrated to cause the focal spot to fall outside this region, or not be formed at all, the wavefront is said to be out of the dynamic range of the sensor. FIG. 3 shows an example of a Shack-Hartmann wavefront sensor 300 in an out-of-range condition.
In practice Shack-Hartmann wavefront sensors have a much greater dynamic range than interferometric sensors. This range may be tens to hundreds of waves of optical path difference. However, this dynamic range is still insufficient to characterize many real optics.
Other Wavefront Sensing Technologies
Other sensors such as the Moire Deflectometer have a higher dynamic range, but lack the sensitivity necessary for accurate measurement of most transmissive optical elements.
Both optical and computational methods have been used to extend the dynamic range of wavefront sensing devices. Some example computational methods include Spot Tracking, Phase Unwrapping, and Angular Spectrum Propagator Reconstruction.
Spot Tracking
This method extends the dynamic range of the Shack-Hartmann wavefront sensor in the case where the wavefront being measured starts out within range and then drifts out of range over a period of time. This case exists for many optical configurations where a lens moves within the optical setup or a component changes optical characteristics due to some cause such as material heating or deformation. Spot tracking is accomplished by comparing current positions of focal spots to positions recorded in a previous frame. The previous positions are used as a starting point for locating the spots after an incremental movement. As long as the frames are taken frequently enough, then it is computationally simple to keep track of them. This technique has been known since at least 1993 (A. Wirth, A Jankovics, F. Landers, C. Baird, and T. Berkopec, xe2x80x9cFinal report on the testing of the CIRS telescopes using the Hartmann technique,xe2x80x9d Tech. Rep. NAS-31786, Task 013 (Adaptive Optics Associates, Cambridge, Mass. 1993)). A limitation to this approach is that the incident wavefront must start out within range.
Phase Unwrapping
In this technique the focal spot to lenslet mapping is inferred using techniques similar to those used in interferometry. This technique is described in xe2x80x9cDynamic range expansion of a Shack-Hartmann sensor by use of a modified unwrapping algorithm,xe2x80x9d by J. Pfund, N. Lindlein, and J. Schwider, Optical Society of America, 1998.
Angular Spectrum Propagator Reconstruction
Described by xe2x80x9cAlgorithm to increase the largest aberration that can be reconstructed from Hartmann sensor measurements,xe2x80x9d by M. Roggemann and T. Shulz, Applied Optics, Vol 37, No 20, 1998, this technique is computationally expensive and therefore inappropriate for many measurement applications.
While there are means for tracking and adjusting the positions of these AOIs (as described previously), the simplest, most robust calculations are achieved for the case where a single mapping of lenslets onto the pixels can be maintained. For example, U.S. Pat. No. 5,825,476 discloses a method that uses a missing focal spot to identify the central AOI, and then tracks all the other focal spots using this missing data. However, if there is a speck of dust on the part under test, this easily fools the identification of this missing spot, leading to inaccurate results.
U.S. Pat. No. 6,550,917, the entire contents of which are hereby incorporated by reference for all purposes as if fully set forth herein, discloses a means for extending the dynamic range of a sensor by adjusting the spherical radius of curvature of a reference sphere to match the effective defocus of the optical system under test (in that case, an eye). While a similar scheme can be applied to testing transmissive optics, the requirements for testing an eye en vivo are significantly different from those of measuring a fixed lens or optical element. For an intraocular lens, the focal length in air can be as short as 10 mm, necessitating the use of a different optical testing method.
Without some tracking scheme, the focal spot moves either completely or partially out of the assigned AOI. Thus the pixels do not accurately represent the focal spot position information. If inaccurate data is used to set the position of an adaptive focal element, then the system may not converge to the correct position, leading to an inaccurate measurement or slow convergence. In a typical embodiment, there may be thousands of potential focal spots. Thus identifying poor focal spots, or focal spots that have wandered outside of their correct AOIs, is useful for maintaining the accuracy of the subsequent calculations.
Accordingly, it would be desirable to provide a system and method for extending the dynamic range of wavefront sensing devices in transmissive optics metrology. It would also be desirable to provide a system and method for sensing and analyzing the wavefront of light passing through an optically transmissive system with enhanced dynamic range. It would be further desirable to provide such a system and method which overcomes one or more disadvantages of the prior art.
The present invention comprises a system and method for sensing and analyzing a wavefront of an optically transmissive system. By analyzing the wavefront, the system and method may ascertain desired parameters of the optically transmissive system. In particular, for example, when the optically transmissive system is a lens such as a contact lens, the system and method may accurately determine the focal length of the lens by sensing and analyzing the wavefront of light passing through the lens. Furthermore, once the focal length has been determined, the system and method may eliminate the lower order focal length term from the analysis, and determine higher order aberrations in the lens.
In one aspect of the invention, a system for sensing a wavefront of light passed through an optical device, comprises: a device under test (DUT) holder adapted to hold the optical device; a point light source adapted to provide light to the optical device; a movable platform adapted to move the point light source along an optical axis of the system; a first lens adapted to receive and pass therethrough a light beam from the optical device; a range-limiting aperture adapted to receive and pass therethrough at least a portion of the light beam from the first lens; a second lens adapted to receive and pass therethrough the portion of the light beam from the range-limiting aperture; a Shack-Hartmann wavefront sensor adapted to receive the portion of the light beam from the second lens and to produce therefrom wavefront data; and a processor adapted to receive the wavefront data from the wavefront sensor and to control movement of the movable platform to move the point light source to a location about one focal length away from the optical device.
In another aspect of the invention, a method of measuring a wavefront of light from an optically transmissive device, comprises: locating a light source a first distance from the optically transmissive device; passing light from the light source through the optically transmissive device; imaging at least a portion of the light passed through the optically transmissive device; sensing a wavefront of the imaged light to produce therefrom wavefront data; and adjusting a location of the light source with respect to the optically transmissive device to substantially maximize a degree of collimation of the light passed through the optically transmissive device.
In yet another aspect of the invention, a system for measuring a wavefront passed through an optically transmissive device, comprises: a light source disposed on a first side of an optically transmissive device; a wavefront sensor disposed on a second side of an optically transmissive device; a relay imaging system disposed between the optically transmissive device and the wavefront sensor; and means for adjusting a distance between the light source and the optically transmissive device.
In still another aspect of the invention, a method of determining a focal length of a lens comprises: (1) locating a light source a first distance from the optically transmissive device; (2) passing light from the light source through the optically transmissive device; (3) imaging at least a portion of the light passed through the optically transmissive device; (4) sensing a wavefront of the imaged light; adjusting a location of the light source with respect to the optically transmissive device to substantially maximize a degree of collimation of the light passed through the optically transmissive device; (5) moving the light source by a distance xi from the location that substantially maximizes the degree of collimation of the light passed through the optically transmissive device, where i=(1, N); (6) sensing a wavefront of the imaged light; (7) calculating a radius of curvature value of the sensed wavefront; (8) repeating the steps (5) through (7) Nxe2x88x921 times (where N is an integer) where the value of xi is changed each time the steps (5) through (7) are repeated; and (9) calculating the focal length of the lens from the N radii of curvature values calculated in the steps (5) though (8).